全文获取类型
收费全文 | 11080篇 |
免费 | 1925篇 |
国内免费 | 1396篇 |
专业分类
化学 | 6521篇 |
晶体学 | 229篇 |
力学 | 527篇 |
综合类 | 83篇 |
数学 | 507篇 |
物理学 | 5453篇 |
无线电 | 1081篇 |
出版年
2024年 | 34篇 |
2023年 | 132篇 |
2022年 | 224篇 |
2021年 | 314篇 |
2020年 | 380篇 |
2019年 | 371篇 |
2018年 | 353篇 |
2017年 | 364篇 |
2016年 | 484篇 |
2015年 | 424篇 |
2014年 | 480篇 |
2013年 | 1286篇 |
2012年 | 623篇 |
2011年 | 720篇 |
2010年 | 574篇 |
2009年 | 701篇 |
2008年 | 676篇 |
2007年 | 620篇 |
2006年 | 654篇 |
2005年 | 588篇 |
2004年 | 526篇 |
2003年 | 509篇 |
2002年 | 450篇 |
2001年 | 332篇 |
2000年 | 396篇 |
1999年 | 302篇 |
1998年 | 219篇 |
1997年 | 216篇 |
1996年 | 221篇 |
1995年 | 148篇 |
1994年 | 170篇 |
1993年 | 141篇 |
1992年 | 123篇 |
1991年 | 105篇 |
1990年 | 80篇 |
1989年 | 69篇 |
1988年 | 57篇 |
1987年 | 36篇 |
1986年 | 48篇 |
1985年 | 43篇 |
1984年 | 47篇 |
1983年 | 10篇 |
1982年 | 24篇 |
1981年 | 22篇 |
1980年 | 16篇 |
1979年 | 14篇 |
1978年 | 10篇 |
1977年 | 9篇 |
1976年 | 7篇 |
1973年 | 24篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
171.
We report a comprehensive theoretical study on the decarbonylation of acetaldehyde by Fe+ and Cr+. Various intermediates, transition states, and products involved in the decarbonylation reactions are fully optimized at the B3LYP/6-311+G(2df,2pd) level of theory. The potential energy surfaces (PESs) corresponding to [M,O,C2,H4]+(M=Cr and Fe) are examined in detail using B3LYP and CCSD(T) methods, respectively. The validity of these theoretical methods is calibrated with respect to the available thermochemical data. Calculations suggest that the Cr+ mediated decarbonylation of acetaldehyde takes place in four steps on the sextet surface: encounter complexation, C-C activation, aldehyde H-shift, and nonreactive dissociation, in good accordance with the Co+ mediated decarbonylation of acetaldehyde [Zhao, Zhang, Guo, Wu, Lu, Chem. Phys. Lett. 2005, 414, 28], while for the Fe+/acetaldehyde system decarbonylation can occur on both the quartet and the sextet PESs. The quartet pathway, which experiences spin-orbit coupling between the two surfaces, is energetically more favorable; whereas along the sextet decarbonylation coordinate several high-energy barriers are revealed. The theoretical results are compared with the experimental product kinetic energy and angular distributions of decarbonylation of acetaldehyde by Fe+ and Cr+ measured using a crossed-beam technique [Sonnenfroh, Farrar, J. Am. Chem. Soc. 1986, 108, 3521]. 相似文献
172.
173.
Ruiz E Rodríguez-Fortea A Cano J Alvarez S Alemany P 《Journal of computational chemistry》2003,24(8):982-989
The application of theoretical methods based on the density functional theory with hybrid functionals provides good estimates of the exchange coupling constants for polynuclear transition metal complexes. The accuracy is similar to that previously obtained for dinuclear compounds. We present test calculations on simple model systems based on H. He and CH(2). He units to compare with Hartree-Fock and multiconfigurational results. Calculations for complete, nonmodeled polynuclear transition metal complexes yield coupling constants in very good agreement with available experimental data. 相似文献
174.
Muhammad Ilyas Sarwar Sonia Zulfiqar Zahoor Ahmad 《Colloid and polymer science》2007,285(15):1733-1739
Fluoro-aramid-based sol/gel-derived nanocomposites were synthesized by condensing a mixture of 4,4′-(hexafluoro-isopropylidene)dianiline
and 1,3-phenylenediamine with terephthaloylchloride (TPC) in dimethylacetamide. TPC was added in slight excess to produce
amide chains with carbonyl chloride end groups and then replaced with alkoxy groups using aminophenyltrimethoxysilane to develop
bonding with the silica network. Mechanical, dynamic mechanical thermal, water absorption and morphological measurements were
carried out on the thin hybrid films. Increase in the tensile strength and modulus was observed as compared to pristine polyamide.
The thermal decomposition temperature was found in the range of 400–500 °C. The water absorption was found to be reduced with
higher silica content. The glass transition temperature and the storage moduli increased with increasing silica concentration.
The maximum increase in the T
g value (345 °C) was observed with 20 wt% silica. Scanning electron microscopy revealed the uniform distribution of silica
in the matrix with an average particle size ranging from 8 to 50 nm. 相似文献
175.
176.
Effects of metal surfaces, such as aluminium, silver and gold, on the melting behavior and thermal polymerization of long-chain diynoic acids having a diacetylene group at different positions were investigated by thermal analyses using DSC, TG and other methods. The surface effects of metals were significant in the order of Ag, Al and Au. These effects are attributable to the anchoring of carboxyl group on the surface by chemisorption, which leads to unfavorable condition for polymerization of heptadeca-2,4-diynoic acid. In the case of tricosa-10,12-diynoic acid, containing a flexible methylene chain, inserted between COOH and C≡C?C≡C groups, the anchoring of COOH on the metal surface causes rather favorable effect on the polymerization. 相似文献
177.
J. K. Percus 《Journal of statistical physics》1986,42(5-6):921-930
A model classical fluid is constructed by assuming that the direct correlation functionc(r – r) is independent of any applied external field. Thermodynamic consistency requires thatc(r – r) 0, and permits explicit representation of the model by a many-body interaction potential. In the canonical ensemble, the model shows a phase transition to an infinite density condensed phase, but in the grand canonical ensemble only an anomalous transition to zero density vapor is found to stably exist. 相似文献
178.
N. Angelescu M. Bundaru G. Costache C. J. Thompson 《Journal of statistical physics》1986,43(1-2):33-49
A generalizedO(n) matrix version of the classical Heisenberg model, introduced by Fuller and Lenard as a classical limit of a quantum model, is solved exactly in one dimension. The free energy is analytic and the pair correlation functions decay exponentially for all finite temperatures. It is shown, however, that even for a finite number of spins the model has a phase transition in then limit. The transition features a specific heat jump, zero long-range order at all temperatures, and zero correlation length at the critical point. The Curie-Weiss version of the model is also solved exactly and shown to have standard mean-field type behavior for all finiten and to differ from the one-dimensional results in then limit. 相似文献
179.
A low-temperature expansion for the free energy density of lattice animals is derived. Analysis of the series yields a collapse transition temperature ofT
c - 0.54, in close agreement with previous estimates. It is demonstrated that
p,k, the number ofp-particle,p-bond animals, obeys the asymptotic scaling law log
p,k
pg(k/p) + o(p). The low-temperature series and numerical data are used to estimate the scaling function. 相似文献
180.
The spherical-model limitn of then-vector model in a random field, with either a statistically independent distribution or with long-range correlated random fields, is studied to demonstrate the correctness of the replica method in which then and replica limits limits are interchanged, provided the replica and thermodynamic limits are taken in the right order, in the case of long-range correlated random fields. A scaling form for the two-point correlation function relevant to the first-order phase transition below the lower critical dimensionality of the random system is also obtained. 相似文献